It has long been recognized that acrylonitrile polymers, and co-polymers containing acrylonitrile, produce films, fibers and other articles having desirable properties. Such products of particular interest are barrier films for use in product packaging. It has also long been recognized that certain aspects of polyacrylonitrile ("PAN") products need to be improved. First, there is a need for improved and easier processing. Secondly, there is a need for improved and more complete removal of the unreacted acrylonitrile monomer during processing of the polyacrylonitrile, because the presence of monomer is undesirable in food and drug packaging products. Thirdly, there is a need to improve the barrier properties of acrylonitrile polymers and co-polymers. Fourthly, it is desirable that strength properties, particularly tensile strength, of acrylonitrile polymers and co-polymers be increased.
Acrylonitrile polymers composed of 90 weight percent or more of acrylonitrile monomer are generally not melt processable. When heated, they char before the melt can be processed at temperatures commonly used in plastics processing equipment.
Shaped articles made from such polymers are presently limited to the production of fibers by dissolving such polymers in a polar solvent, forcing the resulting "syrup" through a spinnerette to form fibers, coagulating them in a heated atmosphere (dry spinning) or in a fluid bath (wet spinning), tensilizing by stretching, and drying by heat volatilization. Solvent vapors are condensed, purified and reused. Such processing is termed "solution" or "solvent" casting.
Experimental acrylonitrile homopolymer and copolymer thin films have been made by solution casting as described in U.S. Pat. No. 3,437,717 to Isley et al. A melt extrusion blown film process is disclosed in U.S. Pat. No. 4,536,365 to Zwick, and in U.S. Pat. No. 4,144,299 to Inoue et al. Another melt reaction-extrusion process utilizing a water leaching system to remove the solvent component prior to biaxially stretching such formed film is described in U.S. Pat. No. 4,066,731 to Hungerford. Film property data taken from such acrylonitrile films show them to have many desirable mechanical, physical, chemical and electrical values for multipurpose use applications. Such films are clear, tough, resistant to ultraviolet radiation, have good temperature resistance, provide high barrier to gas and moisture transmission, and have good dimensional stability.
It appears that as the level of acrylonitrile monomer in such films is increased relative to other monomers, property values also increase. This is especially true of a 99.6 percent acrylonitrile monomer content polymerized with a polyalkenyl monomer serving as a cross-linking agent, as in U.S. Pat. No. 3,437,717. It is also apparent that a small amount of cross-linking agent incorporated into a polyacrylonitrile polymer for solution spinning filaments, yarns and fibers significantly enhances the properties of such end products, as disclosed in U.S. Pat. No. 3,268,490 to Sunden, et al. Conversely, the presence of minor amounts, as little as 1 to 2 percent, of comonomers can frequently reduce the desirable characteristics of the polyacrylonitrile product to a point that little advantage is gained over competitive polymer products.
Among the undesirable aspects of these polyacrylonitrile films is that the level of unpolymerized acrylonitrile monomer can exceed the 0.1 parts per million now allowed under United States Food and Drug Administration regulations. Other undesirable aspects relate to levels of residual and possibly toxic solvent, and the implications associated with packaging of food and drug products, and the higher costs and hazards of recovering, condensing, purifying and storing the volumes of solvent required.
The acrylonitrile polymer having the highest barrier and other property values in a biaxially stretched film is the type of polymer found in U.S. Pat. Nos. 3,437,717 and 3,380,949 to Isley et al., composed of 99.6 weight percent acrylonitrile monomer, as well as in U.S. Pat. No. 3,268,490 to Sundun et al., including copolymers composed of 85 weight percent acrylonitrile monomer. However, the molecular weight of these polymers increases after polymerization, thus causing decreasing rates of dissolution from 17 weight percent to less than 5 weight percent in the solvent system over the course of just 15 days from the date of polymerization. This indicates polymer instability. It is felt that these are major factors which have prevented such experimental films from becoming commercial products.
The disclosures of the above referenced patents are incorporated herein by reference.
Thus, it is an object of this invention to provide improved methods of processing polyacrylonitrile and to provide improved and new PAN compositions and products. This objective, as well as other desirable objectives, are achieved by the invention described and claimed herein.